Description:TITLE OF INVENTION

METHOD FOR CONTROLLED INCREASE OF A BLADE-TOWER-CLEARANCE OF A WIND TURBINE

FIELD OF INVENTION

The present invention is directed to a method for controlled increase of a blade tower clearance of a wind turbine; a controller for a wind turbine configured to execute the method for controlling a wind turbine and said wind turbine.

BACKGROUND

The blade-tower-clearance has to be guaranteed under all conditions, especially during extreme wind situations. It is part of the trade-off between cost and loads and power. The blade-tower-clearance is determined by the blade deflection. A reduction especially in front of the tower when the blade is crossing the tower helps to design a cheaper tower, in particular because of more tower-diameter re-duces needed tower-mass. Stiffer blades, larger cone/tilt-angles or more over-hang of rotor are design-possibilities to increase that clearance. However, these measures either increases costs or decreases annual energy production (AEP).

Beside those design-possibilities control-features can help to increase the clear-ance. Several controlling methods are known in the prior art, which are directed to a cyclic pitching out of blades while passing the tower. That increases the pitch-activity and cost some AEP. A better balance was achieved by introducing wind-speed-dependency.

OBJECT OF THE INVENTION

One object of the present invention is for controlled increase tower clearance with minimum energy loss. At the same time, this increasing of the blade-tower-clearance should be realized without design-change, without AEP-loss and with less as necessary increase of pitch-activity as well as with tolerable very small load-increase.

SUMMARY OF THE INVENTION

For solving the above-mentioned object the present invention based on the idea of a combination of detection and reaction is used. The method is summarized as “Blade Bending Control” (BBC). This consists of two parts, namely detection and reaction.

According to the present invention, the method for controlled increase of blade tower clearance of a wind turbine comprising:
- measuring blade bending moment of at least one rotor blade and
- determining rotor angle F, wherein
- determining a threshold value for blade bending moment or edge-bending moment or flap-bending moment or out-of-plane-bending mo-ment,
- comparing the measured blade bending moment or determined edge-bending moment or determined flap-bending moment or out-of-plane-bending moment with respective determined threshold value, if the threshold value is exceeded then creating trigger signal for setting a wind turbine controller in BBC-mode and
- pitching at least one rotor blade to pitch offset angle a for increasing tower clearance near to the tower.

In context of the present invention at rotor angle F 360° (=0°) one rotor blade is in front of the tower. In other words, the blade is in so-called 6 o’clock position. Accordingly, the other blades are staggered at 120° respectively 240°. Further-more, near to the tower means a rotor angle F between the rotor angle Fstart where the reaction phase starts and the rotor angle Fend where the reactions phase ends.

According to the invention it was recognized that pitching out the blades at tower is important if blade-loads indicate a danger of too high blade-deflection at tower. An additional logic of location of load-exceedance, trigger & hold-time for detec-tion and apply of rotor-angle-based pitch-offsets provides best increase of blade-tower-clearance to enable a cost-reduced tower. Thus, the invented BBC-method allows reducing the blade-bending in front of tower in extreme wind situations by using blade-moment sensors. This results in a reduction of cost for tower by possi-ble increasing tower-diameter with no appreciable negative impact onto AEP, pitch activity and loads. Further no additional sensors or hardware-add-on is nec-essary.

In an advantageously embodiment of the method for controlling a wind turbine, based on the measured blade bending moment an edge-bending moment and/or flap-bending moment will be determined. In this case, a threshold value for the edge-bending moment and/or flap-bending moment will be determined instead of determining a threshold value of the measured blade bending moment. In a next step the determined edge-bending moment and/or flap-bending moment will be compared with the determined threshold value of the edge-bending moment and/or flap-bending moment.

In a further advantageously embodiment of the method for controlling a wind turbine, based on the edge-bending moment and flap-bending moment an out-of-plane bending moment will be calculated. In this case the threshold value for the out-of-plane bending moment will be determined instead of determining a thresh-old value of the blade bending moment. In a next step the calculated out-of-plane bending moment will be compared with the determined threshold value of the out-of-plane bending moment.

A preferred embodiment of the method for controlled increase of blade tower clearance of a wind turbine provides a reaction phase for setting a pitch offset a of at least one rotor blade conducted before said rotor blade passes the tower at rotor angle F 360°.

Advantageously, reaction phase is between the rotor angle Fstart where the pitch offset a starts and the rotor angle Fend where the pitch offsets a ends at 0°. More advantageously, the set-point for pitch-offset a is 0° is send latest at rotor angle F 360°. However, this doesn’t mean that the pitch procedure is finished at this time. It needs time from sending the set-point to reaction pitching the blade and during this time the blade rotates further. Also because of the aerodynamic properties of the blade the moving of said blade needs time. But exactly because of this time delay, it is important to start and finish the reaction phase due time before the blade passes the tower so that the maximal blade tower clearance is reached.

A preferred embodiment of the method for controlled increase of blade tower clearance of a wind turbine provides the threshold value for out-of-plane-bending-moment indicates a minimum distance between tower and rotor blade tip.

Advantageously, the minimum distance between tower and rotor blade tip de-pends on the properties of the rotor blade and the chosen safety factor. The mini-mum distance maybe calculated according to IEC 61400-1 E3.Under any circum-stances the tip tower clearance must not undercut this limit.

A preferred embodiment of the method for controlled increase of a blade tower clearance of a wind turbine provides starting a reaction phase after creating a trig-ger signal.

A more preferred embodiment of the method for controlled increase of a blade tower clearance of a wind turbine provides a reaction phase takes place in a period rotor angle Fp of 180°, in particular 150°, in particular 100° or in particular 70°.

In context of the present invention period rotor angle Fp is between the rotor an-gle Fend where the reaction phase ends and the rotor angle Fstart where the reaction phase starts. In other words, period rotor angle Fp is the difference from the rotor angle Fend where the reaction phase ends and the rotor angle Fstart where the reac-tion phase starts. Advantageously applies here that the smaller the period rotor angle Fp the smaller is the loss of annual energy production. More advantageously, the reaction phase will be conducted during the period rotor Fp.

A preferred embodiment the method for controlled increase of a blade tower clearance of a wind turbine provides the pitch offset a is up to 2°, in particular up to 1.5°, in particular up to 1°, in particular up to 0.5°.

A preferred embodiment of the method for controlled increase of a blade tower clearance of a wind turbine provides holding BBC-mode active for at least a given time after last trigger signal. Advantageously, the BBC-mode will be hold for 20 s, especially 10 s, more especially 5 s. Depending on the rotor speed the BBC-mode should hold e.g. up to 3 rotor turnings. Herewith it should be avoided that during a gust the BBC-mode ends which would increase the danger that the rotor blade hits the tower.

A preferred embodiment of the method for controlling a wind turbine for con-trolled increase blade tower clearance provides a reaction phase comprises:
- first phase, wherein the rotor angle F is before start rotor angle Fstart or end rotor angle Fend and current the pitch offset a is 0°;
- second phase, wherein the rotor angle F between start rotor angle Fstart and first inflection point-rotor angle Ffip, and current the pitch offset a increases from 0° to maximal pitch offset a;
- third phase, wherein the rotor angle F is between first inflection point-rotor angle Ffip and second inflection point-rotor angle Fsip, and current the pitch offset a is maximal and
- fourth phase, wherein the rotor angle F between second inflection point-rotor angle Fsip and end rotor angle Fend, and the current pitch offset a decreases form maximal pitch offset a to 0°.

In context of the present invention the start rotor angle Fstart is defined as the rotor angle F where the pitch offset a starts. The end rotor angle Fend is defined as the rotor angle F where the pitch offset a ends at 0°. The area between the start rotor angle Fstart and end rotor angle Fend is defined as the period rotor angle Fp. Fur-thermore, the first inflection point-rotor angle Ffip is the rotor angle F after start where the pitch offset a reaches maximum after increasing and the second inflec-tion point-rotor angle Fsip is the rotor angle F after start where the pitch offset a is maximal, but decreasing of the pitch offset a begins. In particular, during an in-creasing of the pitch offset a, this can be calculated with equation:

pitch offset a = sin((rotor angle F – start rotor angle Fstart) / LX * p) * amplitude

wherein LX is the minimum of period rotor angle Fp and 40*amplitude of pitch offset a and wherein the amplitude is the maximal pitch offset a.

And in particular, during a decreasing of the pitch offset a, this can be calculated with the equation:

pitch offset a = sin((start rotor angle Fstart + period rotor angle Fp – rotor angle F) / LX * p) * amplitude of pitch offset a

wherein LX is the minimum of period rotor angle Fp and 40*amplitude of pitch offset a and wherein the amplitude is the maximal pitch offset a.

Advantageously, the end rotor angle Fend is at rotor angle F 360°. This means that the reaction phase ends at rotor angle F 360° (the rotor blade is in 6’clock posi-tion) and the pitch offset a is 0°. More advantageously, also here applies that the set-point for pitch-offset a is 0° is send latest at rotor angle F 360°. More advan-tageously, the reaction phase will be repeated for every rotor blade near the tower.

A preferred embodiment of the method for controlled increase of a blade tower clearance of a wind turbine provides the trigger signal of a for-running rotor blade will be considered by the at least one following rotor blade.

Advantageously, the transfer of the trigger signal of the for-running rotor blade to the following rotor blade allows said rotor blade enough time to react.

A preferred embodiment of the method for controlled increase of a blade tower clearance of a wind turbine provides filtering of measured rotor blade bending moment signals.

A further aspect of the invention is directed to a controller of a wind turbine.

The controller for a wind turbine is configured to execute said method for con-trolled increase of a blade tower clearance of a wind turbine as described above.

A preferred embodiment of the controller for a wind turbine provides a filter ele-ment for filtering of measured rotor blade bending moment signals. Advanta-geously, the filter element is a PT1-filter

A preferred embodiment of the controller for a wind turbine provides a BBC-mode, wherein the BBC-mode contains a reaction phase.

A further aspect of the invention is directed to a wind turbine having a controller configured to execute said method for controlled increase of a blade tower clear-ance of a wind turbine as described above.

The wind turbine having a controller for executing said method for for controlled increase of a blade tower clearance of a wind turbine as described above.

The wind turbine is configured to perform the method for controlling a wind tur-bine for controlled increase of a blade tower clearance as described above.

A further aspect of the invention is directed to a computer program product.

The computer program product, which enables a data processing device, once the computer program product is executed on the data processing device, and is pref-erably stored in a storage device, to perform a method of controlling a wind tur-bine as described above.

A further aspect of the invention is directed to a method for operating a wind tur-bine configured to perform the method for controlling a wind turbine for con-trolled increase of a blade tower clearance as described above and/or having a con-troller configured to execute said method for controlled increase of a blade tower clearance as described above.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be explained in more detail with respect to exemplary em-bodiments with reference to the enclosed drawings, wherein:

Figure 1 shows a wind turbine;

Figure 2 shows a diagram for detecting of BBC;

Figure 3 shows a method according to the invention

Figure 4 shows a diagram of the trigger signal and pitch offset and

Figure 5 shows a diagram the reaction phase

The foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with the accompany-ing drawing figures.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 depicts a schematic view of a wind turbine 1 with a tower 2 and a nacelle 3. Depending on given requirements the wind turbine 1 can be used for offshore or onshore applications. The nacelle 3 is rotatable mounted on the tower 2. The nacelle 3 incorporates a number of components of a drive train chain 4 comprising a rotor shaft (not shown) for example. The nacelle 3 also incorporates a generator (not shown) connected with a plurality of electrical components (not shown), which are described in detail later. Further the nacelle 3 comprises a yaw system (not shown) for rotating the nacelle 3. Said rotor shaft is connected to a rotor 5. The rotor 5 comprises three rotor blades 6 which are mounted to a hub body (not shown). Latter is connected to the rotor shaft of the drive train chain 4. The rotor blades 6 are adjustably mounted on the hub body. This is realized by means of pitch drives 8, said pitch drives 8 being part of a pitch system (not shown). The pitch system controls the rotor speed to given set points. By means of pitch-drives 8, the rotor blades 6 may be moved about a rotor blade 6 axes into different pitch positions, said rotor blade 6 axis extending in an axial direction of the rotor blades 6. Each rotor blade 6 is connected to the hub body via its blade bearings. The na-celle 3 is covered by a nacelle cover 9, which has a nacelle cover interface 10. The hub body is covered by a spinner 11, wherein the hub body and spinner 11 form-ing a hub 7. Each rotor blade 6 comprises at least one blade bending sensor 17. For example at each inside of the rotor blade 6 is arranged a flap-bending sensor 17a and an edge-bending sensor 17b. In this case, the wind turbine controller 12 re-ceives signals from the flap-bending sensor 17a and the edge-bending sensor 17b. Furthermore, each blade comprises a pitch angle sensor 28 for measuring the cur-rent pitch angle.

Figure 2 depicts a diagram for detecting of BBC by a wind turbine controller 12. The rotor angle F will be obtained by means for obtaining rotor position 13, may-be arranged near the rotor 5. The rotor position signal will be provided to the wind turbine controller 12. This comprises beside others a blade-position-detection-element 14 for detecting the rotor angle F of each rotor blade 6. The position of the rotor blades 6 will be provided to a blade-down-detection-element 15. In par-ticular at rotor angle F 360° (=0°) the rotor blade 6 is in front of the tower 2. In other words, the blade is in so-called 6 o’clock position. Accordingly, the other rotor blades 6 are staggered at 120° respectively 240°. The signal that the rotor blade 6 is down means at rotor angle 360° will be send to a blade-down-identification-element 16 of the respective rotor blade 6. In described embodiment rotor blade no. 1 of the three rotor blades 6 is down so the blade-down-identification-element 16-1 will be active. Same applies for rotor blade no. 2 or 3 if one of them is down.

Further, the wind turbine controller 12 receives blade bending moments obtained by at least one blade-bending-sensor 17 individually for each of the rotor blades 6. Maybe the at least one blade-bending-sensor 17 is arranged inside of each of the rotor blades 6. As blade-bending-sensor 17 can be used any suitable sensor which are known from prior art. For example at each inside of the rotor blade 6 is ar-ranged a flap-bending sensor 17a and an edge-bending sensor 17b. In this case, the wind turbine controller 12 receives signals from the flap-bending sensor 17a and the edge-bending sensor 17b. Furthermore, each blade comprises a pitch angle sensor xx for measuring the current pitch angle. The sensed signals will be provid-ed to a PT1-filter 18 for filtering of measured rotor blade bending moment signals. After this, the blade bending moments will be forwarded to the out-of-plane-bending-moment-calculation-element 19 where the out-of-plane-bending-moment for the respective rotor blade 6 will be calculated. This calculation is well-known in prior art. The calculated out-of-plane-bending-moments will be forwarded to a compare-element 20, which compares the calculated out-of-plane-bending-moment for the respective rotor blade 6 with a predetermined threshold value for an out-of-plane-bending-moment. A determination of the predetermined threshold value for out-of-plane-bending-moment depends on the design of the rotor blade 6 and the minimal distance between the rotor blade tip and the tower 2. If the predetermined threshold value for out-of-plane-bending-moment will be exceeded by the calculated out-of-plane-bending-moment then a trigger signal will be creat-ed and send to an and-connector-element 21. For example the trigger signal will be send to the respective and-connector-element 21-1, 21-2, 21-3 of the rotor blade no. 1, 2, 3.

If the requirements are met that the respective rotor blade 6 is down and the calcu-lated out-of-plane-bending-moment exceeds the predetermined threshold value for out-of-plane-bending-moment then the trigger-element 22 will be activated. The threshold value is an indicator for the minimum distance between tower 2 and blade tip. The trigger-element 22 creates a trigger signal which is send over a mon-oflop-element 23 to a BBC-activation-element 24. Consequently, the wind turbine controller 12 operates in BBC-mode. The monoflop-element 23 causes that the trigger signal will be hold for a given time. In result also the wind turbine control-ler 12 will be operate in BBC-mode for the same time. For example the BBC-mode will be hold for 20 s, especially 10 s, more especially 5 s. Depending on the rotor speed the BBC-mode should hold e.g. up to 3 rotor turnings. Herewith it should be avoided that during a gust the BBC-mode ends which would increase the danger that the rotor blade 6 hits the tower 2. It should be pointed out that the monoflop-element 23 will be activated and the given time begins running again every time if the requirements are met that a rotor blade 6 is down independent whether rotor blade no. 1, 2, 3 is down and the calculated out-of-plane-bending-moment exceeds the predetermined threshold value for out-of-plane-bending-moment. This will explained in detail with Figure 4.

While the wind turbine controller 12 operates in BBC-mode pitch offset values will be sent to a pitch controller 25 of one of the rotor blade 6 when the respective rotor blade 6 is near the tower. This will be explained in detail with Figure 5 be-low.

Figure 3 depicts the method steps of the method according to the present inven-tion. The method for controlling a wind turbine 1 for controlled increase blade tower clearance, wherein the method comprises:

First step S1 contains measuring blade bending moment of at least one rotor blade 6, especially of all rotor blades 6, via a blade-bending-sensor 17. Of course in case the blade bending moment of all rotor blades 6 will be measured all rotor blades 6 comprises a blade-bending-sensor 17.

Second step S2 contains determining the rotor angle F via a blade-position-detection-element 14.

Third step S3 contains calculating out-of-plane-bending-moment of the at least one rotor blade 6 based on measured blade bending moment and the current pitch angle.

Fourth step S4 contains determining a threshold value for out-of-plane-bending-moment, wherein the threshold value for out-of-plane-bending-moment indicates a minimum distance between tower 2 and rotor blade tip.

Fifth step S5 contains comparing the calculated out-of-plane-bending-moment with determined threshold value, if the threshold value is exceeded then creating trigger signal for setting controller in BBC-mode, wherein starting a reac-tion phase after creating a trigger signal via the trigger-element 22.

Sixth step S6 contains pitching at least one rotor blade 6 to pitch offset angle a for increasing tower clearance near to the tower 2, wherein a reaction phase for setting a pitch offset a of at least one rotor blade 6 conducted before said rotor blade 6 passes the tower 2 at rotor angle F 360°. During the reaction phase a pitch offset a takes place in a period rotor angle Fp of 180°, in particular 150°, in particular 100° or in particular 70°. The pitch offset a is up to 2°, in particular up to 1.5°, in partic-ular up to 1°, in particular up to 0.5°. Further BBC-mode will be hold for at least a given time after last trigger signal. For example the BBC-mode will be hold for 20 s, especially 10 s, more especially 5 s. Depending on the rotor speed the BBC-mode should hold e.g. up to 3 rotor turnings. The trigger signal of a for-running rotor blade 6 will be considered by the at least one following rotor blade 6.

A computer program product that may be stored on a non-volatile storage of the control system may comprise instructions which, when executed by one or more computers e.g. the wind turbine controller 12 cause the one or more computers to carry out the steps of the aforementioned method.

Figure 4 depicts a diagram of the trigger signal 26 and pitch offset. At axis of or-dinates the pitch offset a is shown from 0° to 1.6°. At the axis of abscissas the time in seconds is shown from -1 s to 11 sec. The trigger signal 26 is indicated by a solid line. The trigger signal 26 is created at time 0 s. In result the pitch offset a 27, indicated by a dashed line, is set to maximum of 1.5°. After 7 seconds of the trigger signal 27 the pitch offset a 27 begins to decrease. The pitch offset a 27 will fall until 0° within 3 seconds. In case of the trigger signal 26 will be set again with in the 10 seconds then the time begins again so that it is guaranteed that between setting maximum pitch offset a 1.5° and pitch offset 0° 10 seconds will pass. The trigger signal 27 for a for-running rotor blade 6 will be considered by the at least one following rotor blade 6.

Figure 5 depicts a diagram of the reaction phase. At axis of ordinates the pitch offset a is shown from 0° to 1.5°. At the axis of abscissas the rotor angle F is shown from 280° to 370°. The reaction phase comprises four phases, namely a first phase hereinafter called out of range phase, which is indicated by a solid line, a second phase hereinafter called increasing phase, which is indicated by a dashed line, a third phase hereinafter called middle phase, which is indicated by a dotted line, and a fourth phase hereinafter called decreasing phase, which is indicated by a dash-dotted line.

The out of range phase characterized in that the rotor angle F is before start rotor angle Fstart or after end rotor angle Fend and current the pitch offset a is 0°. The rotor angle F between the start rotor angle Fstart and after end rotor angle Fend is called as period rotor angle Fp. According to the shown embodiment the start ro-tor angle Fstart is 290° and the end rotor angle Fend is 360°. As mentioned above, at 360° the rotor blade 6 is down in so called 6 o’clock position. According to this, the period rotor angle Fp is 70°.

The increasing phase characterized in that the rotor angle F is between start rotor angle Fstart and first inflection point-rotor angle Ffip, and the current the pitch off-set a increases from 0° to maximal pitch offset a. The first inflection point-rotor angle Ffip can be calculated with the equation

first inflection point-rotor angle Ffip = start rotor angle Fstart + ½ LX

wherein LX is the minimum of period rotor angle Fp and 40*amplitude, wherein the amplitude is the maximal pitch offset a (in this embodiment 1.5°). According to this, first after inflection point-rotor angle Ffip is 320°. Further, at the increasing phase the pitch offset a can be calculated with the equation:

pitch offset a = sin((rotor angle F - start rotor angle Fstart) / LX * p) * amplitude of pitch offset a

wherein LX is the minimum of period rotor angle Fp and 40*amplitude, wherein the amplitude is maximal pitch offset a (in this embodiment 1.5°).

The middle phase characterized in that he rotor angle F is between first inflection point-rotor angle Ffip and second inflection point-rotor angle Fsip, and the current the pitch offset a is maximal at 1.5°. According to the shown embodiment, the second inflection point-rotor angle Fsip can be calculated with equation:

second inflection point-rotor angle Fsip = start rotor angle Fstart + period rotor angle Fp - ½ LX

wherein LX is the minimum of period rotor angle Fp and 40*amplitude, wherein the amplitude is maximal pitch offset a (in this embodiment 1.5°). According to this, the second inflection point-rotor angle Fsip is 330°.

The decreasing phase characterized in that the rotor angle F is between second after inflection point-rotor angle Fsip and end rotor angle Fend, and the current pitch offset a decreases form maximal pitch offset a to 0°. According to the shown embodiment, the pitch offset a can be calculated with equation:

pitch offset a = sin((start rotor angle Fstart + period rotor angle Fp – rotor angle F) / LX * p) * amplitude

wherein LX is the minimum of period rotor angle Fp and 40*amplitude, wherein the amplitude is maximal pitch offset a (in this embodiment 1.5°).

This reaction phase will be repeated for every rotor blade 6 and each cycle within the given time after the trigger signal as explained above (see Fig. 4).
LIST OF REFERENCE SIGNS

1 wind turbine
2 tower
3 nacelle
4 drive train chain
5 rotor
6 rotor blades
7 hub
8 pitch drive
9 nacelle cover
10 interface
11 spinner
12 wind turbine controller
13 means for detecting rotor position
14 blade-position-detection-element
15 blade-down-detection-element
16 blade-down-identification-element
16-1 blade-down-identification-element for rotor blade no. 1
16-2 blade-down-identification-element for rotor blade no. 2
16-3 blade-down-identification-element for rotor blade no. 3
17 blade-bending-sensor
17a flap-bending sensor
17b edge-bending sensor
18 PT1-filter
19 out-of-plane-bending-moment-calculation-element
20 compare-element
21 and-connector-element
21-1 and-connector-element for rotor blade no. 1
21-2 and-connector-element for rotor blade no. 2
21-3 and-connector-element for rotor blade no. 3
22 trigger-element
23 monoflop-element
24 BBC-activation-element
25 pitch controller
26 trigger signal
27 pitch offset
28 pitch angle sensor

, Claims:We Claim:

1. A method for controlled increase of a blade tower clearance of a wind tur-bine, wherein the method comprises:
- measuring blade bending moment of at least one rotor blade (6) and
- determining rotor angle F
characterized in that
- determining a threshold value for blade bending moment,
- comparing the measured blade bending moment with respective deter-mined threshold value, if the threshold value is exceeded then creating trigger signal (26) for setting wind turbine controller (12) in BBC-mode, and
- pitching at least one rotor blade (6) to pitch offset angle a for increasing tower clearance near to the tower (2).

2. The method for controlled increase of a blade tower clearance of a wind tur-bine according to claim 1, wherein a reaction phase for setting a pitch offset a of at least one rotor blade (6) conducted before said rotor blade (6) passes the tower (2) at rotor angle F 360°.

3. The method for controlled increase of a blade tower clearance of a wind tur-bine according to claim 1 or 2, wherein the respective threshold value indi-cates a minimum distance between tower (2) and rotor blade tip.

4. The method for controlled increase of a blade tower clearance of a wind tur-bine according to one of the claims 1 to 3, wherein starting a reaction phase after creating a trigger signal (26).

5. The method for controlled increase of a blade tower clearance of a wind tur-bine according to claim 4, wherein during the reaction phase a pitch offset a takes place in a period rotor angle Fp of 180°, in particular 150°, in particular 100° or in particular 70°.

6. The method for controlled increase of a blade tower clearance of a wind tur-bine according to one of the claims 1 to 5, wherein the pitch offset a is up to 2°, in particular up to 1.5°, in particular up to 1°, in particular up to 0.5°.

7. The method for controlled increase of a blade tower clearance of a wind tur-bine according to one of the claims 1 to 6, wherein holding BBC-mode for at least a given time after last trigger signal (26).

8. The method for controlled increase of a blade tower clearance of a wind tur-bine according to one of the claims 1 to 7, wherein a reaction phase comprises:
- first phase, wherein the rotor angle F is before start rotor angle Fstart or end rotor angle Fend and current the pitch offset a is 0°;
- second phase, wherein the rotor angle F between start rotor angle Fstart and first inflection point-rotor angle Ffip, and current the pitch offset a increases from 0° to maximal pitch offset a;
- third phase, wherein the rotor angle F is between first inflection point-rotor angle Ffip and second inflection point-rotor angle Fsip, and current the pitch offset a is maximal and
- fourth phase, wherein the rotor angle F between second inflection point-rotor angle Fsip and end rotor angle Fend, and the current pitch offset a decreases form maximal pitch offset a to 0°.

9. The method for controlled increase of a blade tower clearance of a wind tur-bine according to one of the claims 1 to 8, wherein the trigger signal (26) of a for-running rotor blade (6) will be considered by the at least one following ro-tor blade (6).

10. The method for controlled increase of a blade tower clearance of a wind tur-bine according to one of the claims 1 to 9, wherein filtering of measured rotor blade bending moment signals.

11. A controller for a wind turbine configured to execute the method for con-trolled increase of a blade tower clearance of a wind turbine (1) according to one of the claims 1 to 10.

12. The controller for a wind turbine according to claim 11, characterized by a filter element for filtering of measured rotor blade bending moment signals.

13. The controller for a wind turbine according to claims 11 or 12, characterized by a BBC-mode, wherein the BBC-mode contains a reaction phase.

14. A wind turbine, characterized by a controller according to one of the claims 11 to 13.

15. A wind turbine, characterized in that the wind turbine (1) is configured to perform the method for controlled increase of a blade tower clearance accord-ing to one of the claims 1 to 10.

16. A computer program product, which enables a data processing device, once the computer program product is executed on the data processing device, and is preferably stored in a storage device, to perform a method of controlling a wind turbine according to one of the claims 1 to 10.

17. A Method for operating a wind turbine according to claim 14 or 15.